How to add "convert to jpg" to quick actions menu?

The Convert Image option is missing from that list because it is handled as a system service or an automation workflow.

Here is how you can easily enable or create this quick action to convert HEIC to JPG directly inside Finder:

• Check the Services Menu: Sometimes it is hidden under a different settings layer. Go to System Settings > Keyboard > Keyboard Shortcuts. Click on Services in the left sidebar, expand the Picturesfolder, and check if Convert Image is listed there. If it is, toggle the checkbox on to make it appear in your right-click menu.
• Create it via the Automation App: If the native action is completely gone after your system update, you can build a one-click shortcut in less than a minute. Open the built-in Shortcuts application on your computer.

1. 1. Click the plus icon to create a New Shortcut.
   2. In the right sidebar, search for the action named Convert Image and drag it to the main workspace. Set it to convert input images to JPEG.
   3. In the right-hand shortcut details menu, check the box that says Use as Quick Action and select Finder.
   4. Name your shortcut Convert to JPG and save it. It will instantly show up in your quick actions list.

You posted in the macOS Sonoma community, yet your profile stll shows macOS Big Sur.

If you are indeed on macOS Sonoma (or later) then by right-clicking on a .heic image in the Finder, you can select from the secondary menu: Quick Actions -> Convert Image. It will give you the option of HEIF to JPG and a checkbox to preserve the image metadata. This is automatically enabled by the operating system, so nothing for you to enable in System Settings.

I neither remember, nor do I still have Big Sur installed to confirm if this feature was available in it. If not, then you will need to roll your own solution using Automator.